In conventional computer systems, a plurality of terminals are generally connected via a network around a host computer. Thereby, the host computer can realize concentrated processes. In such a conventional system, a circuit switching network and packet switching network according to a standard protocol X.25 has been introduced.
Since the reliability of the circuit switching was left in hands of the line provider, in order to have backup of the server computers, it was necessary to previously make registration of a calling destination in the event of a fault to a line control apparatus.
Moreover, the backup system according to X.25 is realized on the basis of the concept of transfer of terminated calls as can be seen in X.25 of CCITT (Consultative Committee for International Telephony and Telegraphy). In the case of transfer of terminated calls, a list of the transfer destinations to be used for transfer when the termination side is defective, or when there is a busy condition, is previously provided at the time of extending a virtual circuit depending on X.25. Thereby, transfer is attempted in the sequence of the list until the virtual circuit is normally extended according to the list of such transfer destination and communication can be realized.
On the other hand, in the current trend of down-sizing, computer systems have gradually shifted from a system for executing centralized processes with a large size computer to a distribution type server-client system consisting of a server computer formed from a work station, or the like, and a client computer mainly formed from a personal computer, or the like.
Moreover, as the connectivity of a network is assured with the same protocol not only in the LAN (Local Area Network) but also in WAN (Wide Area Network) by means of an IP (Internet Protocol) based network such as the internet, a large scale server-client system utilizing the same protocol network explained above can be established. Also, recently, portable type computers are frequently connected to such a server-client system for the purpose of communication and various discussions are now being made for IP for mobile communication which is used for moving the portable type computer in order to connect it to different networks.
FIG. 13 illustrates the structure of a network utilizing an IP for mobile communication of the related art. In FIG. 13, 1300, 1310 and 1320 designate LANs; 1301, 1311 and 1321 designate routers; 1302, 1303, 1304 and 1305 designate clients; and 1312 designates the client for mobile communication. The addresses are assigned in the network utilizing the IP for mobile communication of the related art. When the client 1312 for mobile communication is moved to LAN 1320 from LAN 1310, and the client for the mobile communication 1312 has an identification address corresponding to the existing IP address and a virtual address which is set virtually in the network of the transfer destination.
In the existing network system, regarding the address of the client 1312 for mobile communication, the identification address and virtual address are both indicated as "b.a" when the client is connected to LAN 1310 and the address "b.a" is used for communication between the client 1312 for mobile communication and clients 1302 to 1305. When the client 1312 for mobile communication is moved and connected to LAN 1320, the address which is not used in LAN 1320, for example, "c.a" can be set as the virtual address. When a packet for address "b.a" is transmitted from the clients 1302 to 1305, the routers 1301, 1311 and 1321 transmit the packet to the client 1312 for mobile communication moved to LAN 1320 by transferring the packet to the address "b.a" to the address "c.a". Thus, when client 1312 for mobile communication is moved to a different LAN segment, communication can be made between the client 1312 for mobile communication and the clients 1302 to 1305 without changing the setting of the IP address of the client 1312 for mobile communication in the side of the clients 1302 to 1305.
On the other hand, in a large scale server-client system structured as explained above, a plurality of server computers have usually been duplicated to provide against the faults occurring in the server computers, by selectively using two adjacent server computers as the working or backup server computers. The working server and backup server explained above have been connected with an exclusive bus or network, but a plurality of server computers for working server and backup server must be located at the position of the same LAN segment. In addition, according to a certain method, the working server is switched to the backup server in the distant area by allocating the backup server in the distant area. But in this method, the transfer of terminated calls or the method similar to such transfer of terminated calls has been employed.